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Andreas Cellarius, Harmonia Macrocosmica, plate 19 The renewed interest for the exploration of our natural satellite opens the way for a series of experiments useful to constrain its formation, evolution and present composition. Improved satellite models of the lunar gravitational field will give a better global view of its internal structure; selenoseismology, with in-situ measurements, can complement this picture. An accelerometer like ISA (Italian Spring Accelerometer) can contribute in a fundamental way in supporting both types of measurements – in-orbit and on- ground – improving the quality of the range data by a removal of non-gravitational perturbations in the first case, and making a direct measurement of seismic waves in the second case. Both scenarios are discussed in the framework of possible future missions. Iafolla, V. and Nozzoli, S., Plan. Space Sci. 49, 1609 (2001) Iafolla, V. et al., Adv. Space Res. 38, 639 (2006) Iafolla, V. et al., Celest. Mech. Dyn. Astr. 97, 165 (2007) Abstract Some references

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…and how to measure it A measurement of the gravitational field can be obtained by an high-precision tracking of a spacecraft orbiting around it. The different harmonic terms of the field induce perturbations on the orbit that can be used via a numerical integration and orbit determination procedure to get an estimate of the harmonic coefficients. The quality of the determination depends however on the modeling of spacecraft dynamics: this does not always reach the requirements. http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant Measuring their effects directly can substantially help the gravity field determination!

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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario ISA accelerometer ISA is a three-axis accelerometer with an intrinsic noise of 9.810 -10 m s -2 Hz -1/2 in the frequency band of 310 -5 – 10 -1 Hz. It enables a fine measurement of the perturbations of non-gravitational origin acting on a spacecraft. The displacements of a proof–mass due to a perturbing acceleration are detected by means of capacitive transducers in a bridge configuration, followed by a low noise amplifier The capacitive bridge (CB) is biased at high frequency ( f p = 10 kHz ) so that accelerations at frequencies f s produce their unbalance At the output of the CB the signals are seen as a modulation of the bias voltage at the two side–bands f = f p f s in the frequency domain

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Due to the characteristics of the instrument, measuring its performances on-ground (subject to Earth gravity field) is not easy. Thermal and seismic noise constitute a sort of background that could be removed employing two accelerometers in a differential configuration: Common Mode Rejection. The attained level of rejection in the relevant bandwidth is remarkable. It is an important reference for future development. The experience gained in developing the accelerometer for the BepiColombo mission can be used for applications in a number of scenarios in Solar System exploration.

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Istituto Nazionale Di AstrofisicaIstituto di Fisica dello Spazio Interplanetario ISA around the Moon… There is space for improvement! An analysis of recent lunar gravity field models shows that its knowledge at high orders can be improved. ISA can support this improvement. Photo Apollo 11